1. Technical Field
This application relates to a molding material mixture for foundry purposes, comprising a mold sand, a sodium hydroxide aqueous solution, a binding agent based on alkali silicate and additives. This application also relates to a molded part intended for foundry purposes and produced by using the molding material mixture. This application further relates to a process of producing a molded part.
2. Background Information
Molding material mixtures of the initially mentioned type are known from German patent application DE 102004042535 A1 (AS LÜNGEN GmbH) for example, wherein the binding agent is used in the form of an alkali water glass in connection with a particle-shaped metal oxide, for example silicon oxide, aluminum oxide, titanium oxide or zinc oxide in order to improve the strength of casting molds both immediately after molding and precipitation and also after storage and exposure to an increased amount of air humidity. The particle size of the metal oxides preferably amounts to less than 300 μm. According to the examples, the screen residue on a screen with a mesh width of 63 μm amounts to less than 10 percent by weight, preferably less than 8 percent by weight.
A further process of producing molding material mixtures whose purpose it is to achieve a high strength when combined with a polyphosphate- or borate-containing binding agent is described in U.S. Pat. No. 5,641,015. In column 4, line 39 of the U.S. patent it is mentioned that, as a result of a drying process making use of polyphosphate- or borate-containing binding agent, there is released water which is absorbed by adding silicon dioxide in the finest possible particles. This silicon dioxide consists of porous primary particles which are produced by a precipitation process, which particles comprise a grain size ranging between 10 and 60 nm, and which are agglomerated into secondary particles with a particle size of several μm (column 3, lines 64-66 of the U.S. patent).
An inorganic binding agent system for molding materials is described in European Patent 1095719B1, according to which, in the case of a binding agent based on alkali silicate with added sodium hydroxide aqueous solution, it is possible to improve the flow resistance by adding 8-10 percent by mass with reference to the binding agent. This improvement was accompanied by a higher moisture content of the core sand.
In addition to prior art measures of improving the strength value, more particularly the bending strength of molded parts, it is necessary to take into account further influencing factors which determine the quality of a molding material mixture, as discussed below.
Most importantly, it is necessary to mention flowability, which is known as a significant parameter for the suitability of the molding material when filling a core shooting machine.
Further important parameters are the precipitation curve and the reduction in sensitivity to air humidity.
However, the main quality characteristic to be achieved by the molding material mixture is the surface quality of the casting. Unfortunately, under the conditions prevailing in mass production, the prior art processes are not sufficiently stable, so that again and again, the reject quotas and the unacceptable additional costs due to the need for re-treatment are too high. The most suitable standard for assessing the surface quality has been found to be the determination of the surface percentage of sand adhesions on the casting.